CBSE Class 12 · 2002 · All India Compartment · Compounds Containing NitrogenShort Answer

As the internal salt ${}^{+}{\mathrm{H}}_3\mathrm{N}-{\mathrm{C}}_6{\mathrm{H}}_4-\mathrm{S}{\mathrm{O}}_3^{-}$ the molecule binds itself in a high-melting ionic lattice that plain water cannot break.
In aqueous base: the –NH${}_3^{+}$ loses its proton → the soluble anion H${}_2$N–C${}_6$H${}_4$–SO${}_3^{-}$.
In aqueous acid: the –SO${}_3^{-}$ is protonated → the soluble cation ${}^{+}$H${}_3$N–C${}_6$H${}_4$–SO${}_3$H.